The present invention relates to a microporous film and the use thereof as a separator in batteries.
Modern devices require an energy source, such as primary or secondary batteries, which enable them to be used irrespective of their spatial context. The disadvantage of primary batteries is that they have to be disposed of. As a result, an increasing number of storage (secondary) batteries are being used, which can be charged up again and again using a mains battery charger. Nickel-cadmium batteries (NiCd batteries), for instance, can achieve a service live of approximately 1000 charge cycles if used correctly.
Primary and secondary batteries always consist of two electrodes, which are immersed in an electrolyte solution, and a separator, which separates the anode from cathode. The different types of secondary battery are distinguished by the electrode material used, the electrolyte and the separator used. During charging, a current flows through the battery. The flow of current triggers an electrochemical reaction at the electrodes. Once the battery is charged, it can supply current until the chemical reaction, which is the reverse of the charging process, is exhausted.
The purpose of a battery separator is to provide a spatial division between the anode and cathode in primary batteries and the negative and positive electrode in storage batteries. The separator must be a barrier that isolates the two electrodes from one another electrically, in order to avoid short-circuits. At the same time, however, the separator must be permeable to ions, so that electro-chemical reactions can take place in the cell.
A battery separator must be thin, so that the internal resistance is as low as possible and a high packing density can be achieved. This is the only way of achieving good performance data and high capacities. In addition, it is necessary for the separators to absorb the electrolyte and guarantee ion exchange when the cells are full. Whereas such things as fabric were used previously, nowadays predominantly fine-pored materials such as non-woven fabrics and membranes are used.
Just as there are different battery systems, the separators used in them must differ too, e.g. according to the electrolyte to which they are exposed during their service life. A further criterion for the choice of separator is price. Separators that remain stable over many charge and discharge cycles are made from higher-grade materials than those used in cheaper disposable batteries.
The occurrence of short-circuits is a problem, particularly in lithium batteries. In the case of thermal loading, the battery separator may melt in lithium ion batteries, leading to a short-circuit with disastrous consequences. Similar risks exist if the lithium batteries suffer mechanical damage or are overcharged due to a defect in the charger's electronic system.
In order to increase the safety of lithium ion batteries, shut-down membranes were developed in the past. These special separators close their pores in the shortest time at a given temperature, which is significantly lower than the melting point or ignition point of lithium. The catastrophic consequences of a short-circuit in lithium batteries are thereby largely avoided.
At the same time, though, the separators also need to have a high mechanical strength, which is guaranteed by materials with high melting temperatures. Hence, for instance, polypropylene membranes are advantageous due to their good puncture resistance, but polypropylene's melting point of around 164° C. is very close to lithium's flash point (170° C.)
The prior art discloses how polypropylene membranes can be combined with further layers, which are constructed from materials with a lower melting point, such as polyethylene. Such modifications of the separators must not, of course, have a detrimental effect on the other properties, such as porosity, or provide an added impediment to ion migration. However, the inclusion of polyethylene layers has a very negative effect on the permeability and mechanical strength of the separator overall. In addition, the adhesion of the polyethylene layers to polypropylene is problematic, with the result that only selected polymers in these two classes can be coextruded.